Check if running task is null in onPrepareGestureEndAnimation

- The crash happen when swiping to overview from home without any task in recents
- A simple null check fixes the issue

Fix: 194166137
Test: FallbackRecentsTest#goToOverviewFromHome
Change-Id: I9344a96a4a78fcfc88d2ad53e44ecd43b23114d5
diff --git a/quickstep/src/com/android/quickstep/views/RecentsView.java b/quickstep/src/com/android/quickstep/views/RecentsView.java
index b8a38ad..e128942 100644
--- a/quickstep/src/com/android/quickstep/views/RecentsView.java
+++ b/quickstep/src/com/android/quickstep/views/RecentsView.java
@@ -1792,7 +1792,7 @@
                 .displayOverviewTasksAsGrid(mActivity.getDeviceProfile())) {
             TaskView runningTaskView = getRunningTaskView();
             float runningTaskPrimaryGridTranslation = 0;
-            if (indexOfChild(runningTaskView) != getNextPage()) {
+            if (runningTaskView != null && indexOfChild(runningTaskView) != getNextPage()) {
                 // Apply the gird translation to running task unless it's being snapped to.
                 runningTaskPrimaryGridTranslation = mOrientationHandler.getPrimaryValue(
                         runningTaskView.getGridTranslationX(),